Torque vectoring axle assembly

ABSTRACT

A drive axle assembly includes first and second axleshafts connected to a pair of wheels and a drive mechanism operable to selectively couple a driven input shaft to one or both of the axleshafts. The drive mechanism includes a differential, a speed changing unit operably disposed between the differential assembly and the first and second axleshafts, first and second mode clutches and a brake unit. The first mode clutch is operable to increase the rotary speed of the first axleshaft which, in turn, causes a corresponding decrease in the rotary speed of the second axleshaft. The second mode clutch is operable to increase the rotary speed of the second axleshaft so as to cause a decrease in the rotary speed of the first axleshaft. The brake unit is operable to engage the speed changing unit. A control system controls actuation of both mode clutches.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/852,620 filed on May 24, 2004.

FIELD OF THE INVENTION

The present invention relates generally to differential assemblies foruse in motor vehicles and, more specifically, to a differential assemblyequipped with a torque vectoring drive mechanism and an active controlsystem.

BACKGROUND OF THE INVENTION

In view of consumer demand for four-wheel drive vehicles, many differentpower transfer system are currently utilized for directing motive power(“drive torque”) to all four-wheels of the vehicle. A number of currentgeneration four-wheel drive vehicles may be characterized as includingan “adaptive” power transfer system that is operable for automaticallydirecting power to the secondary driveline, without any input from thevehicle operator, when traction is lost at the primary driveline.Typically, such adaptive torque control results from variable engagementof an electrically or hydraulically operated transfer clutch based onthe operating conditions and specific vehicle dynamics detected bysensors associated with an electronic traction control system. Inconventional rear-wheel drive (RWD) vehicles, the transfer clutch istypically installed in a transfer case for automatically transferringdrive torque to the front driveline in response to slip in the reardriveline. Similarly, the transfer clutch can be installed in a powertransfer device, such as a power take-off unit (PTU) or in-line torquecoupling, when used in a front-wheel drive (FWD) vehicle fortransferring drive torque to the rear driveline in response to slip inthe front driveline. Such adaptively-controlled power transfer systemcan also be arranged to limit slip and bias the torque distributionbetween the front and rear drivelines by controlling variable engagementof a transfer clutch that is operably associated with a centerdifferential installed in the transfer case or PTU.

To further enhance the traction and stability characteristics offour-wheel drive vehicles, it is also known to equip such vehicles withbrake-based electronic stability control systems and/or tractiondistributing axle assemblies. Typically, such axle assemblies include adrive mechanism that is operable for adaptively regulating theside-to-side (i.e., left-right) torque and speed characteristics betweena pair of drive wheels. In some instances, a pair of modulatableclutches are used to provide this side-to-side control, as is disclosedin U.S. Pat. Nos. 6,378,677 and 5,699,888. According to an alternativedrive axle arrangement, U.S. Pat. No. 6,520,880 discloses ahydraulically-operated traction distribution assembly. In addition,alternative traction distributing drive axle assemblies are disclosed inU.S. Pat. Nos. 5,370,588 and 6,213,241.

As part of the ever increasing sophistication of adaptive power transfersystems, greater attention is currently being given to the yaw controland stability enhancement features that can be provided by such tractiondistributing drive axles. Accordingly, this invention is intended toaddress the need to provide design alternatives which improve upon thecurrent technology.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide adrive axle assembly for use in motor vehicles which is equipped with anadaptive yaw control system.

To achieve this objective, the drive axle assembly of the presentinvention includes first and second axleshafts connected to a pair ofwheels and a torque distributing drive mechanism that is operable fortransferring drive torque from a driven input shaft to the first andsecond axleshafts. The torque distributing drive mechanism includes adifferential, a speed changing unit, first and second mode clutches anda brake unit. The differential includes an input component driven by theinput shaft, a first output component driving the first axleshaft and asecond output component driving the second axleshaft. The speed changingunit includes a first shaft driven by the input component, a secondshaft and a gearset for changing the rotary speed of the second shaftrelative to the first shaft. The first mode clutch is operable forselectively coupling the first output component of the differential tothe second shaft. Likewise, the second mode clutch is operable forselectively coupling the second output component of the differential tothe second shaft. The brake unit is operable for selectively engagingthe gearset. Accordingly, selective control over actuation of one orboth of the first and second mode clutches and the brake unit providesadaptive control of the speed differentiation and the torque transferredbetween the first and second axleshafts. A control system including andECU and sensors are provided to control actuation of both mode clutchesand the brake unit.

According to one preferred embodiment, the speed changing unit of thetorque distributing drive mechanism is an overdrive unit that isoperable to increase the rotary speed of the second shaft relative tothe first shaft. As such, engagement of the first mode clutch results inthe first axleshaft being overdriven relative to the second axleshaft.Additionally, engagement of the second mode clutch results in the secondaxleshaft being overdriven relative to the first axleshaft.

According to an alternative preferred embodiment, the speed changingunit of the torque distributing drive mechanism is an underdrive unitthat is operable to decrease the rotary speed of the second shaftrelative to the first shaft. As such, engagement of the first modeclutch results in the first axleshaft being underdriven relative to thesecond axleshaft. In contrast, engagement of the second mode clutchresults in the second axleshaft being underdriven relative to the firstaxleshaft.

Pursuant to an alternative objective of the present invention, thetorque distributing drive mechanism can be utilized in a power transferunit, such as a transfer case, of a four-wheel drive vehicle toadaptively control the front-rear distribution of drive torque deliveredfrom the powertrain to the front and rear wheels.

Further objectives and advantages of the present invention will becomeapparent by reference to the following detailed description of thepreferred embodiments and the appended claims when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a diagrammatical illustration of an all-wheel drive motorvehicle equipped with a drive axle having a torque distributingdifferential assembly and an active yaw control system according to thepresent invention;

FIG. 2 is a schematic illustration of the torque distributingdifferential assembly shown in FIG. 1;

FIG. 3 is another illustration of the torque distributing differentialassembly shown in FIGS. 1 and 2;

FIG. 4 is a diagrammatical illustration of the power-operated actuatorsassociated with the torque distributing differential assembly of thepresent invention;

FIG. 5 is a schematic illustration of an alternative embodiment of thetorque distributing differential assembly of the present invention;

FIG. 6 is a diagrammatical illustration of the torque distributingdifferential assembly of the present invention installed in a powertransfer unit for use in a four-wheel drive vehicle;

FIG. 7 is a schematic drawing of the power transfer unit shown in FIG.6;

FIGS. 8 and 8A are schematic illustrations of another alternativeembodiment of the torque distributing differential assembly of thepresent invention; and

FIG. 9 is a schematic illustration of a modified version of the torquedistributing differential assembly shown in FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an all-wheel drive vehicle 10 includes an engine 12transversely mounted in a front portion of a vehicle body, atransmission 14 provided integrally with engine 12, a front differential16 which connects transmission 14 to front axleshafts 18L and 18R andleft and right front wheels 20L and 20R, a power transfer unit (“PTU”)22 which connects front differential 16 to a propshaft 24, and a rearaxle assembly 26 having a torque distributing drive mechanism 28 whichconnects propshaft 24 to axleshafts 30L and 30R for driving left andright rear wheels 32L and 32R. As will be detailed, drive mechanism 28is operable in association with a yaw control system 34 for controllingthe transmission of drive torque through axleshafts 30L and 30R to rearwheels 32L and 32R.

In addition to an electronic control unit (ECU) 36, yaw control system34 includes a plurality of sensors for detecting various operational anddynamic characteristics of vehicle 10. For example, a front wheel speedsensor 38 is provided for detecting a front wheel speed value based onrotation of propshaft 24, a pair of rear wheel speed sensors 40 areoperable to detect the individual rear wheel speed values based rotationof left and right axle shafts 30L and 30R, and a steering angle sensor42 is provided to detect the steering angle of a steering wheel 44. Thesensors also include a yaw rate sensor 46 for detecting a yaw rate ofthe body portion of vehicle 10, a lateral acceleration sensor 48 fordetecting a lateral acceleration of the vehicle body, and a lock switch50 for permitting the vehicle operator to intentionally shift drivemechanism 28 into a locked mode. As will be detailed, ECU 36 controlsoperation of a pair of mode clutches associated with drive mechanism 28by utilizing a control strategy that is based on input signals from thevarious sensors and lock switch 50.

Rear axle assembly 26 includes an axle housing 52 within which drivemechanism 28 is rotatably supported. In general, torque distributingdrive mechanism 28 includes an input shaft 54, a differential 56, aspeed changing unit 58, a first mode clutch 60 and a second mode clutch62. As seen, input shaft 54 includes a pinion gear 64 that is inconstant mesh with a hypoid ring gear 66. Ring gear 66 is fixed forrotation with a drive case 68 associated with differential 56. As seen,differential 56 is a planetary gearset having an annulus ring gear 70fixed for common rotation with drive case 68, a sun gear 72 fixed forrotation with right axleshaft 30R, a differential carrier 74 fixed forrotation with left axleshaft 30L, and meshed pairs of first planet gears76 and second planet gears 78. First planet gears 76 are shown to bemeshed with sun gear 72 while second planet gears 78 are meshed withannulus ring gear 70. Differential carrier 74 is a multi-piece assemblyhaving a front carrier ring 80 interconnected to a rear carrier ring 82with first and second pins 84 and 86, respectively, extendingtherebetween and on which corresponding first and second planet gears 76and 78 are rotatably supported. Differential 56 is operable to transferdrive torque from drive case 68 to axleshafts 30L and 30R at a ratiodefined by the gear components while permitting speed differentiationtherebetween. Preferably, a 50/50 torque split ratio is established bydifferential 56 for use in this particular drive axle application. Itshould be understood that differential 56 is merely intended torepresent one differential arrangement applicable for use with thepresent invention and that other know planetary and hypoid-typedifferentials could be substituted for use with the present invention.

Speed changing unit 58 includes a gearset having an input sun gear 90,an output sun gear 92, and a plurality of equally-spaced compound gears94. Speed changing unit 58 also includes a first shaft 96 which connectsinput sun gear 90 for common rotation with drive case 68 and a secondshaft 98 which connects output sun gear 92 for common rotation with aclutch drum 100 associated with both first mode clutch 60 and secondmode clutch 62. Compound gears 94 each include a first speed gear 102that is interconnected to a second speed gear 104 via an integral hubsegment 106. Furthermore, first speed gear 102 of each compound gear 94is meshed with input sun gear 90 while its corresponding second speedgear 104 is meshed with output sun gear 92. Compound gears 94 arerotatably supported on pins 108 that are fixed to a support platesegment 110 of housing 52.

In operation, speed changing unit 58 functions to cause a change in therotary speed of second shaft 98 relative to the rotary speed of firstshaft 96. In particular, the speed ratio established between first shaft96 and second shaft 98 is based on the size and number of teeth for eachgear component of speed changing unit 58. In accordance with one firstpreferred arrangement, speed changing unit 58 is an overdrive unit thatis operable to increase the speed of second shaft 98 relative to firstshaft 96, thereby causing a corresponding increase in the rotary speedof clutch drum 100 relative to drive case 68 of differential 56. Toaccomplish such a speed increase, it is contemplated that input sun gear90 could have 27 teeth and output sun gear 92 could have 24 teeth whileboth first speed gear 102 and second speed gear 104 of compound gears 94could each have 17 teeth pursuant to one non-limiting example for speedchanging unit 58.

With continued reference to FIGS. 2 and 3, first mode clutch 60 is shownto be operably disposed between differential carrier 74 and clutch drum100. In particular, a clutch hub 114 of first mode clutch 60 isconnected to rear carrier ring 82 of differential carrier 74 via a thirdshaft 116. As seen, third shaft 116 surrounds right axleshaft 30R whileboth of first shaft 96 and second shaft 98 surround third shaft 116.First mode clutch also includes a multi-plate clutch pack 118 that isoperably disposed between drum 100 and hub 114 and a power-operatedclutch actuator 120. First mode clutch 60 is operable in a first or“released” mode so as to permit unrestricted rotation of second shaft 98relative to third shaft 116. In contrast, first mode clutch 60 is alsooperable in a second or “locked” mode to couple third shaft 116 forcommon rotation with second shaft 98.

As will be recalled, speed changing unit 58 is driven by drive case 68of differential 56 and functions to increase the rotary speed of secondshaft 98. Thus, first mode clutch 60 functions in its locked mode toincrease the rotary speed of differential carrier 74 which, in turn,causes a corresponding increase in the rotary speed of left axleshaft30L. Such an increase in the rotary speed of left axleshaft 30R causesdifferential 56 to drive right axleshaft 30R at a corresponding reducedspeed, thereby directing more drive torque to left axleshaft 30L than istransmitted to right axleshaft 30R. First mode clutch 60 is shiftedbetween its released and locked modes via actuation of power-operatedclutch actuator 120 in response to control signals from ECU 36.Specifically, first mode clutch 60 is operable in its released mode whenclutch actuator 120 applies a predetermined minimum cutch engagementforce on clutch pack 118 and is further operable in its locked mode whenclutch actuator 120 applies a predetermined maximum clutch engagementforce on clutch pack 118.

Second mode clutch 62 is shown to be operably disposed between rightaxleshaft 30R and clutch drum 100. In particular, second mode clutch 62includes a clutch hub 124 that is fixed for rotation with rightaxleshaft 30R, a multi-plate clutch pack 126 disposed between hub 24 anddrum 100, and a power-operated clutch actuator 128. Second mode clutch62 is operable in a first or “released” mode so as to permitunrestricted relative rotation between axleshaft 30R and second shaft98. In contrast, second mode clutch 62 is also operable in a second or“locked” mode to couple axleshaft 30R for common rotation with secondshaft 98. Thus, second mode clutch 62 functions in its locked mode toincrease the rotary speed of right axleshaft 30R which, in turn, causesdifferential 56 to decrease the rotary speed of left axleshaft 30L,thereby directing more drive torque to right axleshaft 30R than isdirected to left axleshaft 30L. Second mode clutch 62 is shifted betweenits released and locked modes via actuation of power-operated clutchactuator 128 in response to control signals from ECU 36. In particular,second mode clutch 62 operates in its released mode when clutch actuator128 applies a predetermined minimum clutch engagement force on clutchpack 126 while it operates in its locked mode when clutch actuator 128applies a predetermined maximum clutch engagement force on cutch pack126.

As seen, power-operated clutch actuators 120 and 128 are shown inschematic fashion to cumulatively represent the components required toaccept a control signal from ECU 36 and generate a clutch engagementforce to be applied to corresponding clutch packs 118 and 126. To thisend, FIG. 4 diagrammatically illustrates the basic components associatedwith such power-operated clutch actuators. Specifically, eachpower-operated actuator includes a controlled device 132, a forcegenerating mechanism 134, and a force apply mechanism 136. Inelectromechanical systems, controlled device 132 would represent suchcomponents as, for example, an electric motor or an electromagneticsolenoid assembly capable of receiving an electric control signal fromECU 36. The output of controlled device 132 would drive force generatingmechanism 134 which could include, for example, a ball ramp, a ballscrew, a leadscrew, a pivotal lever arm, rotatable cam plates, etc.,each of which is capable of converting the output of controlled device132 into a clutch engagement force. Finally, force apply mechanism 136functions to transmit and exert the clutch engagement force generated byforce generating mechanism 134 onto clutch packs 118 and 126 and caninclude, for example, an apply plate or a thrust plate. If ahydra-mechanical system is used, controlled device 132 could be anelectrically-operated control valve that is operable for controlling thedelivery of pressurized fluid from a fluid source to a piston chamber. Apiston disposed for movement in the piston chamber would act as forcegenerating mechanism 134. Preferably, controlled device 132 is capableof receiving variable electric control signals from ECU 36 forpermitting variable regulation of the magnitude of the clutch engagementforce generated and applied to the clutch packs so as to permit“adaptive” control of the mode clutches.

In accordance with the arrangement shown, torque distributing drivemechanism 28 is operable in coordination with yaw control system 34 toestablish at a least four distinct operational modes for controlling thetransfer of drive torque from input shaft 54 to axleshafts 30L and 30R.In particular, a first operational mode is established when first modeclutch 60 and second mode clutch 62 are both in their released mode suchthat differential 56 acts as an “open” differential so as to permitunrestricted speed differentiation with drive torque transmitted fromdrive case 68 to each axleshaft 30L, 30R based on the tractiveconditions at each corresponding rear wheel 32L, 32R. A secondoperational mode is established when both first mode clutch 60 andsecond mode clutch 62 are in their locked mode such that differential 56acts as a “locked” differential with no speed differentiation permittedbetween rear axleshafts 30L, 30R. This mode can be intentionallyselected via actuation of lock switch 50 when vehicle 10 is beingoperated off-road or on poor roads.

A third operational mode is established when first mode clutch 60 isshifted into its locked mode while second mode clutch 62 is operable inits released mode. As a result, left axleshaft 30L is overdriven at thesame increased speed as second speed gear 104. As noted, such anincrease in the rotary speed of left axleshaft 30L causes acorresponding speed reduction in right axleshaft 30R. Thus, this thirdoperational mode causes right axleshaft 30R to be underdriven while leftaxleshaft 30L is overdriven when required to accommodate the currenttractive or steering condition detected and/or anticipated by ECU 36based on the particular control strategy used. Likewise, a fourthoperational mode is established when first mode clutch 60 is shiftedinto its released mode and second mode clutch 62 is shifted into itslocked mode. As a result, right rear axleshaft 30R is overdrivenrelative to drive case 68 which, in turn, causes left axleshaft 30L tobe underdriven at a corresponding reduced speed. Thus, this fourthoperational mode causes right axleshaft 30R to be overdriven while leftaxleshaft 30L is underdriven when required to accommodate the currenttractive or steering conditions detected and/or anticipated by ECU 36.

At the start of vehicle 10, power from engine 12 is transmitted to frontwheels 20L and 20R through transmission 14 and front differential 16.Drive torque is also transmitted to torque distributing drive mechanism28 through PTU 22 and propshaft 24 which, in turn, rotatably drivesinput pinion shaft 58. Typically, mode clutches 60 and 62 would benon-engaged such that drive torque is transmitted through differential56 to rear wheels 32L and 32R. However, upon detection of lost tractionat front wheels 20L and 20R, one or both mode clutches 60 and 62 can beengaged to provide drive torque to rear wheels 32L and 32R based on thetractive needs of the vehicles.

In addition to on-off control of the mode clutches to establish thevarious drive modes associated with overdrive connections through speedchanging unit 58, it is further contemplated that variable clutchengagement forces can be generated by power-operated actuators 120 and128 to adaptively regulate the left-to-right speed and torquecharacteristics. This “adaptive” control feature functions to provideenhanced yaw and stability control for vehicle 10. For example, areference yaw rate can be determined based on several factors includingthe steering angle detected by steering angle sensor 42, the vehiclespeed as calculated based on signals from the various speed sensors, anda lateral acceleration as detected by lateral acceleration sensor 48.ECU 36 compares this reference yaw rate with an actual yaw rate valuedetected by yaw sensor 46. This comparison will determine whethervehicle 10 is in an understeer or an oversteer condition so as to permityaw control system 34 to be adaptively control actuation of the modeclutches to accommodate these types of steering tendencies. ECU 36 canaddress such conditions by shifting drive mechanism 28 into the specificoperative drive mode that is best suited to correct the actual oranticipated oversteer or understeer situation. Optionally, variablecontrol of the mode clutches also permits adaptive regulation of theside-to-side torque transfer and speed differentiation characteristicsif one of the distinct drive modes is not adequate to accommodate thecurrent steer tractive condition.

Referring now to FIG. 5, an alternative embodiment of torquedistributing drive mechanism 28 of FIG. 2 is shown and designated byreference numeral 28′. Generally speaking, a large number of componentsare common to both drive mechanism 28 and 28′, with such componentsbeing identified by the same reference numbers. However, drive mechanism28′ is shown to include a modified speed changing unit 58′. Inparticular, speed changing unit 58′ is a speed reducing or “underdrive”gearset which includes an input sun gear 90′, an output sun gear 92′,and compound gears 94′. Each compound gear 94′ includes a first speedgear 102′ meshed with input sun gear 90′ and a second speed gear 104′meshed with output sun gear 92′. An integral hub segment 106′interconnects first speed gear 102′ for common rotation with secondspeed gear 104′. In essence, speed changing unit 58′ is now arranged toreduce the speed of second shaft 98 relative to first shaft 96 at areduction ratio determined by the gear components. To accomplish thisspeed reduction feature, it is contemplated that input sun gear 90′could have 24 teeth and output sun gear 92′ could have 27 teeth whilefirst speed gear 102′ and second speed gear 104′ each still could have17 teeth pursuant to one non-limiting example.

Drive mechanism 28′ is similar but slightly different in operationcompared to drive mechanism 28 in that first mode clutch 60 nowfunctions to cause left axleshaft 30L to be underdriven relative toright axleshaft 30R while second mode clutch 62 functions to cause rightaxleshaft 30R to be underdriven relative to left axleshaft 30L. As such,the four distinct operational modes previously described are againavailable and can be established by drive mechanism 28′ via selectiveactuation of power-operated clutch actuators 120 and 128.

Referring now to FIG. 6, a four-wheel drive vehicle 10′ is shownequipped with a power transfer unit 160 that is operable fortransferring drive torque from the output of transmission 14 to a first(i.e., front) output shaft 162 and a second (i.e., rear) output shaft164. Front output shaft 162 drives a front propshaft 166 which, in turn,drives front differential 16 for driving front wheels 20L and 20R.Likewise, rear output shaft 164 drives a rear propshaft 168 which, inturn, drives a rear differential 170 for driving rear wheels 32L and32R. Power transfer unit 160, otherwise known as a transfer case,includes a torque distributing drive mechanism 172 which functions totransmit drive torque from its input shaft 174 to both of output shafts162 and 164 so as to bias the torque distribution ratio therebetween,thereby controlling the tractive operation of vehicle 10′. As seen,torque distribution mechanism 172 is operably associated with a tractioncontrol system 34′ for providing this adaptive traction control featurefor vehicle 10′.

Referring primarily to FIG. 7, torque distribution mechanism 172 ofpower transfer unit 160 is shown to be generally similar in structure todrive mechanism 28′ of FIG. 5 with the exception that drive case 68 isnow drivingly connected to input shaft 174 via a transfer assembly 180.In the arrangement shown, transfer assembly 180 includes a firstsprocket 182 driven by input shaft 174, a second sprocket 184 drivingdrive case 68, and a power chain 186 therebetween. As seen, front outputshaft 162 is driven by differential carrier 74 of differential 56 whichnow acts as a center or “interaxle” differential for permitting speeddifferentiation between the front and rear output shafts whileestablishing a full-time four-wheel drive mode. In addition, sun gear 72of differential 56 drives rear output shaft 164. Also, hub 124 of secondmode clutch 62 is shown to be coupled for common rotation with rearoutput shaft 164.

Control over actuation of mode clutches 60 and 62 results incorresponding increases or decreases in the rotary speed of rear outputshaft 164 relative to front output shaft 162, thereby controlling theamount of drive torque transmitted therebetween. In particular, whenboth mode clutches are released, unrestricted speed differentiation ispermitted between the front and rear output shafts while the gear ratioestablished by the components of interaxle differential 56 controls thefront-to-rear torque ratio based on the current tractive conditions ofthe front and rear wheels. In contrast, with both mode clutches engaged,a locked four-wheel drive mode is established wherein no interaxle speeddifferentiation is permitted between the front and rear output shafts.Such a drive mode can be intentionally selected via lock switch 50 whenvehicle 10′ is driven off-road or during severe road conditions. Anadaptive full-time four-wheel drive mode is made available under controlof traction control system 34′ to limit interaxle slip and vary thefront-rear drive torque distribution ratio based on the tractive needsof the front and rear wheels as detected by the various sensors. Inaddition to power transfer unit 160, vehicle 10′ could also be equippedwith a rear axle assembly having either torque distributing drivemechanism 28 or 28′ and its corresponding yaw control system, as isidentified by the phantom lines in FIG. 6.

Referring now to FIG. 8, rear axle assembly 26 is shown equipped with adrive mechanism 28A that is generally similar to drive mechanism 28shown in FIG. 2 except that a brake unit 150 is now provided inassociation with speed changing unit 58A. Speed changing unit 58A islikewise generally similar to speed changing unit 58 of FIG. 2 exceptthat compound gears 94 are rotatably supported on pins 108 that are nowfixed to a rotatable carrier assembly 152 instead of being fixed tostationary support plate segment 110 of housing 52. Carrier assembly 152includes a pair of laterally-spaced carrier rings 154 and 156 betweenwhich pins 108 are mounted. Brake unit 150 is shown to be operablydisposed between housing 52 and carrier assembly 152 and includes abrake drum 158 fixed to carrier ring 154 and a power-operated brakeactuator 160 mounted to housing 52. Brake unit 150 is operable in afirst or “released” mode so as to permit unrestricted rotation ofcarrier assembly 152 relative to housing 52. In contrast, brake unit 150is further operable in a second or “braked” mode to prevent rotation ofcarrier assembly 152. Brake unit 150 is shifted between its released andbraked modes via actuation of power-operated brake actuator 160 inresponse to control signals from ECU 36. Specifically, brake unit 150 isoperable in its released mode when brake actuator 160 applies apredetermined minimum brake force on brake drum 158 and is furtheroperable in its braked mode when brake actuator 160 applies apredetermined maximum brake force on brake drum 158.

In accordance with the arrangement shown in FIG. 8, torque distributingdrive mechanism 28A is operable in coordination with yaw control system34 to establish at least five distinct operational modes for controllingthe transfer drive torque from input shaft 54 to axleshafts 30L and 30R.In particular, a first operational mode is established when first modeclutch 60, second mode clutch 62 and brake unit 150 are all in theirreleased mode such that differential 56 acts as an open differential soas to permit unrestricted speed differentiation with drive torquetransmitted from drive case 68 to axleshafts 30L, 30R based on thetraction at wheels 32L, 32R. Moreover, with brake unit 150 released,carrier assembly 152 is not grounded such that drum 100 is not driven bygearset 58A so as to reduce and/or eliminate parasitic losses normallycaused by slip across multi-plate clutch packs 118 and 126. A secondoperational mode is established when first mode clutch 60 and secondmode clutch 62 are in their locked modes and brake unit 150 is in eitherof its released or braked mode such that differential 56 acts as alocked differential with no speed differentiation permitted between rearaxleshafts 30L, 30R.

A third operational mode is established when first mode clutch 60 andbrake unit 150 are shifted into their respective locked and braked modeswhile second mode clutch 62 is operable in its released mode. As aresult, left axleshaft 30L is overdriven at the same increased speed assecond speed gear 104. As noted, such an increase in rotary speed ofleft axleshaft 30L causes a corresponding reduction in the rotary speedof right axleshaft 30R. Thus, left axleshaft 30L is overdriven whileright axleshaft 30R is underdriven to provide a torque vectoring powerdistribution between left and right wheels 32L and 32R. Likewise, afourth operational mode is established when first mode clutch 60 is inits released mode, second mode clutch 62 is in its locked mode and brakeunit 150 is in its braked mode. As a result, right axleshaft 30R isoverdriven relative to drive case 68 which, in turn, causes leftaxleshaft 30L to be underdriven at a corresponding reduced speed.

Finally, a fifth operational mode is available with drive mechanism 28Awhen brake unit 150 is in its released mode, first mode clutch 60 in itslocked mode and second mode clutch 62 is adaptively modulated betweenits released and locked modes to provide an adaptive limit slip functionwithout torque vectoring. Such adaptive limit slip control of the torquetransferred between axleshafts 30L and 30R across differential 56enhances the tractive characteristics of the vehicle. A similar limitedslip feature is also available if first mode clutch 60 is modulatedwhile second mode clutch 62 is locked and brake unit 150 is in itsbraked mode.

Brake unit 150 is schematically shown in FIG. 8 to illustrate a drumbrake type device having brake actuator 160 adapted to mechanicallyengage or electromechanically act on brake drum 158. For example, it iscontemplated that actuator 160 could be generally arranged similar tothat shown in FIG. 4 so as to include a controlled device adapted toreceive control signals from ECU 36 for actuating a force generatingmechanism capable of applying the brake force on brake drum 158. Inaddition, it is optional that brake unit 150 be arranged as a frictionclutch instead of a drum brake device. This alternative arrangement isshown in FIG. 8A wherein a brake unit 150A includes a power-operatedclutch actuator 160A and a multi-plate clutch pack 170 operably disposedbetween brake hub 158A and casing 52.

Referring now to FIG. 9, a “modular” version of drive mechanism 28A fromFIG. 8 is shown and identified by reference numeral 28B. In general,drive mechanism 28B permits the final drive ratio that is transmittedbetween pinion input shaft 54 and drive casing 68 to be easily changedvia the use of an intermediate gearset 200. Gearset 200 includes ahypoid pinion gear 202 meshed with drive pinion 64 which drives atransfer gear 204 which, in turn, is meshed with a drive gear 206 fixedto drive case 68.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A motor vehicle, comprising: a powertrain operable for generatingdrive torque; a primary driveline for transmitting drive torque fromsaid powertrain to first and second primary wheels; a secondarydriveline for selectively transmitting drive torque from said powertrainto first and second secondary wheels, said secondary driveline includingan input shaft driven by said powertrain, a first axleshaft driving saidfirst secondary wheel, a second axleshaft driving said second secondarywheel, and a drive mechanism disposed between said input shaft and saidfirst and second axleshafts, said drive mechanism including adifferential, a speed changing unit, first and second mode clutches anda brake unit, said differential having an input component driven by saidinput shaft, a first output component driving said first axleshaft and asecond output component driving said second axleshaft, said speedchanging unit having a first shaft driven by said input component, asecond shaft and a gearset for disposed between said first and secondshafts, said first mode clutch is operable for selectively coupling saidfirst output component to said second shaft, said second mode clutch isoperable for selectively coupling said second output component to saidsecond shaft, and said brake unit is operable for selectively engagingsaid gearset to permit said second shaft to be driven by said firstshaft; and a control system for controlling actuation of said first andsecond mode clutches and said brake unit.
 2. The motor vehicle of claim1 wherein said drive mechanism is operable to establish a firstoverdrive mode when said first mode clutch is engaged, said brake unitis engaged and said second mode clutch is released for overdriving saidfirst axleshaft relative to said input component such that saiddifferential causes said second axleshaft to be underdriven relative tosaid input component.
 3. The motor vehicle of claim 2 wherein said drivemechanism is operable to establish a second overdrive mode when saidfirst mode clutch is released, said brake unit is engaged and saidsecond mode clutch is engaged for overdriving said second axleshaftrelative to said input component such that said differential causes saidfirst axleshaft to be underdriven relative to said input component. 4.The motor vehicle of claim 1 wherein said drive mechanism establishes alocked mode when both of said first and second mode clutches are engagedand said brake unit is engaged.
 5. The motor vehicle of claim 1 whereinsaid drive mechanism is operable to establish a first underdrive modewhen said first mode clutch is engaged, said brake unit is engaged andsaid second mode clutch is released such that said first axleshaft isunderdriven relative to said input component and said second axleshaftis overdriven relative to said input component.
 6. The motor vehicle ofclaim 5 wherein said drive mechanism is operable to establish a secondunderdrive mode when said first mode clutch is released, said brake unitis engaged and said second mode clutch is engaged such that said secondaxleshaft is underdriven relative to said input component and said firstaxieshaft is overdriven relative to said input component.
 7. The motorvehicle of claim 1 wherein said differential includes a ring gear as itsinput component, a differential carrier as its first output component, asun gear as its second output component, and planet gears supported bysaid differential carrier and which are meshed with said ring gear andsaid sun gear.
 8. The motor vehicle of claim 1 wherein said speedchanging unit includes an input sun gear driven by said first shaft, anoutput sun gear driving said second shaft, and speed gears meshing withsaid input and output sun gears, said speed gears are rotatablysupported from a carrier with said brake unit operable to selectivelybrake rotation of said carrier.
 9. The motor vehicle of claim 8 whereinsaid speed gears include a first gear meshed with said input sun gearwhich is interconnected to a second gear meshed with said output sungear.
 10. The motor vehicle of claim 1 wherein said first mode clutchincludes a first clutch pack disposed between said second shaft and saidfirst output component and a first power-operated clutch actuatoroperable to generate and exert a clutch engagement force on said firstclutch pack, wherein said second mode clutch includes a second clutchpack disposed between said second shaft and said second output componentand a second power-operated clutch actuator operable to generate andexert a clutch engagement force on said second clutch pack, wherein saidbrake unit includes a rotatable brake member coupled to a component ofsaid gearset and a power-operated brake actuator operable to generateand exert a braking force on said brake member, and wherein said controlsystem includes a control unit operable to control actuation of saidfirst and second clutch actuators and said brake actuator.
 11. A driveaxle assembly for use in a motor vehicle having a powertrain and firstand second wheels, comprising: an input shaft driven by the powertrain;a first axleshaft driving the first wheel; a second axleshaft drivingthe second wheel; a drive mechanism coupling said input shaft to saidfirst and second axleshafts, said drive mechanism including adifferential, a speed changing unit, first and second mode clutches anda brake unit, said differential having an input component driven by saidinput shaft, a first output component driving said first axleshaft and asecond output component driving said second axleshaft, said speedchanging unit having a first shaft driven by said input component, asecond shaft and a gearset disposed between said first and secondshafts, said first mode clutch is operable for selectively coupling saidfirst output component to said second shaft, said second mode clutch isoperable for selectively coupling said second output component to saidsecond shaft, and said brake unit is operable for selectively engagingsaid gearset to permit said second shaft to be driven at a differentrotary speed relative to said first shaft; and a control system forcontrolling actuation of said first and second mode clutches and saidbrake unit.
 12. The drive axle of claim 11 wherein said drive mechanismis operable to establish a first overdrive mode when said first modeclutch and said brake unit are engaged and said second mode clutch isreleased such that said first axleshaft is overdriven relative to saidinput component and said second axleshaft is underdriven relative tosaid input component.
 13. The drive axle of claim 12 wherein said drivemechanism is operable to establish a second overdrive mode when saidfirst mode clutch is released and said second mode clutch and said brakeunit are engaged such that said second axleshaft is overdriven relativeto said input component and said first axleshaft is underdriven relativeto said input component.
 14. The drive axle of claim 11 wherein saiddrive mechanism establishes a locked mode when all of said first andsecond mode clutches and said brake unit are engaged.
 15. The drive axleof claim 11 wherein said drive mechanism is operable to establish afirst underdrive mode when said first mode clutch and said brake unitare engaged and said second mode clutch is released such that said firstaxleshaft is underdriven relative to said input component and saidsecond axleshaft is overdriven relative to said input component.
 16. Thedrive axle of claim 15 wherein said drive mechanism is operable toestablish a second underdrive mode when said first mode clutch isreleased and said second mode clutch and said brake unit are engagedsuch that said second axleshaft is underdriven relative to said inputcomponent and said first axleshaft is overdriven relative to said inputcomponent.
 17. The motor vehicle of claim 11 wherein said differentialincludes a ring gear as its input component, a differential carrier asits first output component, a sun gear as its second output component,and planet gears supported by said differential carrier and which aremeshed with said ring gear and said sun gear.
 18. The drive axle ofclaim 11 wherein said speed changing unit includes an input sun geardriven by said first shaft, an output sun gear driving said second shaftand speed gears meshing with said input and output sun gears that aresupported from a rotatable carrier, said brake unit operable toselectively brake rotation of said carrier.
 19. The drive axle of claim18 wherein said speed gears include a first gear meshed with said inputsun gear which is interconnected to a second gear meshed with saidoutput sun gear.
 20. The drive axle of claim 18 wherein said first modeclutch includes a first clutch pack disposed between said second shaftand said first output component and a first power-operated clutchactuator operable to generate and exert a clutch engagement force onsaid first clutch pack, wherein said second mode clutch includes asecond clutch pack disposed between said second shaft and said secondoutput component and a second power-operated clutch actuator operable togenerate and exert a clutch engagement force on said second clutch pack,wherein said brake unit includes a brake member fixed for rotation withsaid carrier and a power-operated brake actuator operable to generateand exert a brake force on said brake member and wherein said controlsystem includes a control unit operable to control actuation of saidfirst and second clutch actuators.
 21. A drive axle assembly for use ina motor vehicle having a powertrain and first and second wheels,comprising: an input shaft driven by the powertrain; a first axleshaftdriving the first wheel; a second axleshaft driving the second wheel; adifferential having a ring gear driven by said input shaft, a sun gearfixed for rotation with said first axleshaft, a differential carrierfixed for rotation with said second axleshaft, and meshed pairs of firstand second planet gears rotatably supported by said differentialcarrier, said first planet gears are meshed with said sun gear and saidsecond planet gears are meshed with said ring gear; a speed changingunit having a second sun gear driven by said ring gear, a third sun gearand compound planet gears rotatably supported from a planet carrier andhaving a first speed gear meshed with said second sun gear and a secondspeed gear meshed with said third sun gear; a first mode clutch forselectively coupling said third sun gear for rotation with saiddifferential carrier; a second mode clutch for selectively coupling saidthird sun gear for rotation with said first axleshaft; a brake unit forselectively braking rotation of said planet carrier; and a controlsystem for controlling actuation of said first and second mode clutchesand said brake unit.